Build fails with gcc-15 / std=c23

[heitbaum]

gcc-15 switched to -std=c23 by default:

https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=55e3bd376b2214e200fa76d12b67ff259b06c212

In file included from common/shell.h:24,
                 from common/cvar.h:26,
                 from common/quakedef.h:33,
                 from common/common.c:34:
common/qtypes.h:37:15: error: expected identifier before 'false'
   37 | typedef enum{ false, true, ENSURE_INT_QBOOL = 0x70000000 } qboolean;
      |               ^~~~~